1. Field of the Invention
The present invention generally relates to a printed wiring board, and more particularly, to a printed wiring board and method of fabricating the same.
2. Description of Related Art
The common printed wiring board is usually fabricated by lamination or build-up process. In the conventional lamination process, a predetermined number of the single-sided boards or the double-sided boards are provided. Wherein, the single-sided board means that a dielectric layer has a single conductive layer formed thereon, and the double-sided board means that a dielectric layer has two conductive layers formed on its two surfaces thereof respectively. Next, the conductive layers of the single-sided boards or the double-sided boards are patterned. Next, uncured resins are stacked between the single-sided boards or the double-sided boards having the patterned conductive layers to form a lamination structure. Finally, conductive through vias are formed in the lamination structure to electrically connect the patterned conductive layers. Thus, a printed wiring board is fabricated by a lamination process.
It should be noted that the uncured resin is subjected to a high temperature bake process to cure the dielectric layers. Accordingly, if the printed wiring board is fabricated by the lamination process, the structure of the printed wiring board must be symmetrical in thickness and number of layers with respect to the uncured resin in order to prevent the dielectric layer made of resin from warping because it shrinks when cured.
Compared to the above-mentioned lamination process, another conventional lamination process uses—the build-up of B-stage dielectric layers and conductive layers on two sides of a core layer sequentially and symmetrically. Next, the conductive layers are patterned to make via openings. Next, a plurality of openings are formed in the dielectric layers by laser, and then the openings are filled with a conductive material to form a plurality of conductive vias, such that the conductive layers on both sides of the dielectric layer can be electrically connected to each other through the conductive vias. Then, the conductive traces are made by patterning the metal layers.
FIG. 1 is a schematic cross-sectional view showing a conventional build-up printed wiring board. Referring to FIG. 1, a conventional metal patterning process is used to form a conductive layer 12 on both sides of a core layer 10 of the dielectric material. First, a plurality of conductive through vias 14 is formed in the core layer 10 to electrically connect the conductive layers 12. Next, the conductive layers 12 formed on both sides of the core layer 10 are patterned. A B-stage dielectric layer 16 with a copper foil 20 is built up on both sides of the core layer of 10 and 12 by a lamination process. Next, a plurality of blind vias 18 is formed in the top and bottom metal foils 20 and dielectric layers 16, respectively, and then a plating step is used to electrically connect layers 12 and 20 through vias 18.
Next, the two conductive layers 20 on the two dielectric layers 16 are patterned. Finally, the above-mentioned dielectric layers 16, the conductive blind vias 18 and the conductive layers 20 are duplicated to form a build-up printed wiring board 22 according to the required number of the conductive layers.
It should be noted that when the B stage dielectric layer is comprised of a thermosetting resin material, the dielectric layer must be subjected to a high temperature bake process in order to cure the dielectric layer. During the curing step, the B-stage dielectric shrinks and causes a stress in the structure. Accordingly, during the build-up process, the patterned conductive layers and the dielectric layers formed on two sides of the core layer must be symmetrical to avoid the dielectric layers from being warped when they are cured. Therefore, an even number of the patterned conductive layers is required. However, it is desirable for applications such as portable electronics to reduce the thickness of the printed wiring board and to increase the integration thereof. Besides, the core layer serves as a carrier plate for supporting the patterned conductive layers and the dielectric layers in the build-up process. However, the core layer has a definite thickness which, together with the build up layers on both sides, may result in a thickness exceeding the maximal allowable value required by some products.